Engineering Rock Mechanics

Questions and answers: excavation principles 259

of the content of these classifications is already indirectly included.

The art in applying the engineering rock mechanics principles to rock

engineering is to adopt a pragmatic approach by successfully capturing

the essence of the problem without introducing unnecessary complica-

tions. The authors of the article have successfully achieved this.

415.8 In relation to rock tunnel boring machines (TBMs), two pro-
gress rates can be defined:
Cutring rarte, C the rate at which a tunnel is constructed when the
tvnnelling machine is operating; and
Advance rate, A: the rate at which a tunnel is constructed.

These two rates are not the same because the TBM is not continu-
ously cutting; there is a Uti/iz&.on factor, U, which is the amount
of time (expressed as a proportion) that the machine is cutting, Le.
A=UxC.
Over a short period, say 20 min, A can be equal to C. Over longer
periods, of days, weeks or months, there can be a greater and
greater discrepancy between C and A, as indicated in the diagram
below (Barton, 1999 5).
Cutting rate, C m/hr Advance rate, A
(log scale) mhr (log scale)
10
?t

1 hr 1 day 1 wk 1 rnth 1 Yr

The terms 'Excellent' through to 'Extremely poor' refer to the rates,

and Barton noted that the majority of TBM tunnels lie between the

'Good' and 'Poor' lines.Giive at least five reasons why such lines

occur, i.e. why the value of U decreases with time.

A758 Five reasons are as follows.
(1) The machine needs periods for scheduled maintenance, cutter re-
placements and repair. As time goes on, the cumulative effect will
be to reduce the value of U because of additional damage events
and failure of machine components.
(2) If the rock needs supporting, it may be necessary to stop the
machine to allow roof and sidewall rock bolting and shotcreting to
take place.

5This diagram was included in the General Report presented by Barton (1999) at

the 9th ISRM Congress on Rock Mechanics held on 25-28 August 1999 in Paris, and

is included in the article Barton N. (1999) TBM Performance Estimation in Rock Using

QTBM. Tunnels and Tunnelling, September 1999,N-34.